1. Field of the Invention
The present invention relates to data processing systems and more particularly to timing apparatus in a communication subsystem for indicating required time intervals.
2. Description of the Prior Art
In many sophisticated data processing systems, it is important to be able to efficiently control the timing of events to insure optimum utilization of the resources. For example, a central processor manually employs an interval timer which enables it to keep track of the time of day and to interrupt periodically as required. Thus, this allows for the performance of functions such as the timeout of events, the polling of communication networks on a regular basis and the time of day recording.
In many prior art systems, it is encumbent upon the processor to respond to a timeout condition within a maximum time period after receipt of a timer interrupt. Waiting can result in the readout of inactivate time information. Accordingly, the processor is required to interrupt normal processing to respond immediately to each timer produced interrupt.
U.S. Pat. No. 4,090,239 entitled "Interval Timer for Use in an Input/Output System" describes a timer unit including a preset register for storing a processor established count for defining a time interval, an interval counter coupled to the preset register for receiving a count therefrom, and a rollover counter coupled to the interval counter. The timer unit upon detecting each completion of the time interval specified by the preset register generates an interrupt request to a priority network. This timing system requires a preset register and a number of counters. Also, a single processor must initiate the count and also respond to a single interrupt signal.
The hardware required is reduced by the use of a microprocessor controlling communication lines through the use of channel control blocks stored in a memory. Such a system is described in U.S. Pat. No. 4,133,030 entitled "Control System Providing for the Transfer of Data in a Communications Processing System Employing Channel Dedicated Control Blocks". This system, however, limited the throughput by restricting the number of communication lines that could be processed.
It should be understood that the references cited herein are those which the Applicants are aware of and are presented to acquaint the reader with the level of skill in the art and may not be the closest reference to the invention. No representation is made that any search has been conducted by the Applicants.